It’s the offseason so I’m still a bit rusty at the video commentary right now, but bare with me.

Tropical Cyclone Funso is making himself known as a big problem for Mozambique in eastern Africa. He stalled out for the last few days on the coastline, and has already dumped 12-18 inches of rain on portions of the country, with more still falling. The storm is now moving southeastward back out over the water, and will eventually pull away enough to let the rain diminish. However, the southern part of Mozambique will likely have to deal with its own share of heavy rain from Funso as the storm makes a close pass to the coastline on its way southward Tuesday through Thursday. Steering currents are very weak in the area, and a weak subtropical ridge building to Funso’s south will try to force the storm back westward towards the African coastline before he is able to recurve to the southeast. It remains to be seen whether there will be a full landfall, but there is a pretty good chance that the storm’s core will affect southern Mozambique. With environmental conditions very favorable at this time for intensification, Funso will likely be a Cat 3 or Cat 4 cyclone, posing a very serious danger to the region.

Funso was spawned as a piece of the very active tropical pattern that we have seen during the last couple of weeks in the Indian Ocean, thanks to abnormally warm waters there for a 2nd-year La Nina. The waters off of western Australia, in particular, are quite hot, running 2-3C above normal right now. Later this week a tropical cyclone is expected to develop there within the monsoon trough and possibly move inland. All of this indicates that the SOI is still strongly positive overall, and it is now back up in the dailies from the brief bout of negative numbers that it put up last week. A positive SOI shows that the La Nina is still around and won’t be going anywhere particularly fast, and we will likely have to wait for the spring season for it to weaken substantially. This will continue to shape the northern hemisphere winter, and will keep the southern hemisphere tropical season active.

The global tropics are fairly quiet overall, with the only activity confined to the southern Indian Ocean, where the remnants of Tropical Cyclone Chanda are beginning to exit Madagascar, leaving much of the island drenched in over a foot of rain in some places. These remnants may get stuck north of a subtropical ridge and try to regenerate later, but for now they will cease to be an immediate threat after leaving Madagascar. An invest off of northwestern Australia may become a weak tropical storm over the next few days as it moves southwards towards the west Australian coastline, bringing rain to that area, but no models strengthen it beyond manageable intensity for now.

The video shows an interesting vertically-stacked low pressure area in the Sea of Okhotsk, which may become a polar low during the next 48-72 hours as it sits nearly stationary and strengthens, in part due to warm-core processes.

Back in North America, the forecast battle for the rest of the winter continues. Blocking is due to develop up through Alaska, Siberia, and over the north pole this week, sending the arctic oscillation negative and providing a tempting look to the pattern for snowlovers in the eastern United States. However, the arctic vortex over northern Baffin Bay is remaining strong, and the blocking over the pole will do little to it except squeeze it slightly more southward and allow the northern tier states of the U.S. to get in on some seasonal cold at times during the next 10 days or so. The southeast and central part of the country will get a day or two of cold as a trough rolls through in about 96 hours, but then normal temperatures will return in its wake.

The thing about the Alaskan blocking is that such blocks loves to build up and then cut-off into blocking highs that retrograde westward into northern Russia as they weaken. Heights then crash in their wake, and the arctic vortex loves to redevelop farther west into the void left by the exiting block. All of the ensemble sets are starting to see this in the 10-15 day forecast, and I show those in the video. This would make sense also because the troughing loves to be in western Canada when the PDO is negative and cold water exists along western North America, helping heights to lower. With the arctic vortex remaining intact and shifting westward during the next two weeks, it is going to be very hard to shut off the strong zonal flow over the U.S. and get true long-lasting cold air into the eastern half of the country. As the cold builds in western Canada, some may get into the northwest U.S. and midwest as time goes on, which is only natural in this kind of a winter, but for the east and southeast, don’t expect a big pattern flip going into the end of January.

The above video explores stratospheric warming events that have occurred since 1979 during El Nino and La Nina winters. We are likely facing a stratospheric warming event forecasted to develop during the rest of January, and it is already being proclaimed as the kick to the bucket that will allow cold to pour into the eastern United States. While the pattern may turn colder overall for the U.S., the video shows how many strato-warming events never shove cold all the way into the eastern U.S. during La Ninas. The reason is that the stratosphere’s influence over the troposphere is not all-powerful, and is often overcome by other things happening within the troposphere itself, such as the PDO and ENSO signals which, when negative, often keep the southeast U.S. ridge intact, allowing warmth to remain. During El Nino winters, however, the Pacific pattern is aligned with what the stratospheric warming tries to do, and all of those events are very cold for the eastern United States.

It is not clear yet whether this upcoming warming event will work its way down to the troposphere and provide arctic blocking, but some models are supporting that idea along with a colder pattern for North America. If this happens, I have a feeling that the models will again overestimate the cold for the rest of the winter in the eastern U.S., and while a battleground may set up between warmth in the southeast and cold trying to invade the midwest and New England, overall the eastern seaboard will remain near normal to slightly above normal, rounding out the rest of the winter. Transient cold shots may still occur, such as the big one that just roared through the east during the last couple of days, bringing freezes to Florida. However, despite the potential development of arctic blocking, there may be more inviting places for the cold air to go than into the eastern United States, though overall the pattern will likely turn colder for the country as a whole.

This would have been a video, and it may still be one later, but it’s hard to do videos when I’m at home, and they may have to wait until school resumes.

It is now 2012, and the heart of the winter is now beginning in earnest. December 2011 defied many forecasts that had significant cold invading the eastern half of the nation, and the ideas presented here that it should be warmer than normal instead turned out to verify nicely, as seen in the CPC analysis below:

January and February now lie before us, and some of the model ensembles, namely the GFS and CFSv2, are showing a gradual breakdown of the positive AO that we have had, allowing blocking to become established over the north pole during the next few weeks. Below are today’s 12z GFS ensemble 500mb mean valid January 19th (left) and the CFSv2 500mb mean Week 4 forecast for the last week of January (right).

A lot of this blocking may be due to a stratospheric warming event that is currently taking place, which may work itself down into the troposphere and produce blocking over the pole. What’s interesting about these model forecasts is that despite this, the arctic vortex doesn’t vanish, but rather simply moves a little bit, namely farther south deeper into Canada. The mean location of the arctic vortex in the winter is right over northern Baffin Bay, and all it has done here is moved southward over Hudson Bay instead. Something is keeping this strong low alive this winter, and so far it has prevented any blocking from developing over Canada or Greenland, and even as the models show the AO going negative, Canada remains a massive cold anomaly at 500mb.

If this indeed is to be the pattern that we see develop during the heart of the winter, I decided to take a look at the past and try to find some analogs that might give us a similar look. I searched for strong negative PDOs (November 2011 had the 3rd-strongest negative PDO for that month on record) with a fading multi-year La Nina and an AO that started positive in early winter but crashed later. Two winters fit the bill nicely: 1956-57 and 1971-72. Below are the PDO and AO values for these winters, with 2011 through November thrown in (December values aren’t in yet, but we know the PDO is still strongly negative and the AO was strongly positive).

And here’s a montage showing 500mb anomalies for the Decembers of 2011, 1956, and 1971:

Notice the similarities to this December, with a strong polar vortex over northeast Canada and ridging over much of the United States bringing warmth to the east and central part of the country. Now if we progress into the heart of the winter and look at the 500mb anomalies during January and February of 1957 and 1972, we see a strong similarity to the CFSv2 and GFS ensemble forecasts for late January:

As blocking develops southwest of Alaska and up into the Arctic Ocean, the polar vortex is shoved southward into Canada, imposing itself upon the northern United States. The fact that it remains strong is important, because it means that the the U.S. won’t necessarily get entirely overrun with cold. Some forecasts now are setting their hopes on this new polar blocking to get the cold into the southern and eastern United States during late January. However, although the arctic vortex would shift southward if this pattern comes to pass, the limitations on how deep the U.S. troughs can get are still there. In December I talked about the angular momentum conservation which makes it hard for monster troughs to develop over the eastern U.S. when there is no blocking to the north. Here the same principle applies, and the flow is likely to want to remain fairly flat most of the time over the CONUS. We still don’t have any blocking showing up over Canada or Greenland on the models, and without that, it’s hard to rock the the eastern U.S. with frigid, snowy weather.

However, this pattern overall would be much colder than what we had in December, and a greater portion of the U.S., mostly the Pacific Northwest and the midwest, would see bitter temperatures at times due to the main jetstream flow that would now be out of Alaska and the north pole instead of off the Pacific Ocean. A fierce battleground would set up between cold to the north and warmth still hanging on in the southeast, causing snow and ice problems for the country’s midsection. We saw this in our two analog years being discussed here: 1957 and 1972. Here are the U.S. temperatures during January and February of both:

Notice that despite the arctic blocking, the south remains warm because Canada is still dominated by low heights aloft. There is far more to winter forecasting than just looking at the AO/NAO. They are indices that describe the pattern, but they do not give the full picture. They are but a shadow of the real thing, and it is the actual real thing that we should be looking at. A negative AO/NAO does not guarantee a cold, snowy eastern winter, as we have learned here from 1957 and 1972. One must look at the entire picture to see what’s going on.

The moral here is that the pattern is changing, likely in response to the recent changes in the stratosphere, and a shift to a generally colder pattern for North America may be on the way. If it does come, the models and analogs are in agreement that the arctic vortex will not be replaced by the blocking, but only shifted southward by it, imposing itself on the warmth in the southern U.S. and generating a battle between the different air masses right over the United States, causing nasty winter weather in many places and probably above-average snowfall in parts of the midwest and northern New England. However, such a pattern would still support normal to slightly above normal temperatures and below-normal snowfall for most of the eastern seaboard and southeast United States, in accordance with my winter forecast. You may recall that it also called for cold in the west and across the northern part of the country, which hasn’t happened in earnest so far except in the southwest, but may start to if the models are correct on this coming pattern change.